You can get current selection from the Solution Explorer and cast it to ProjectItem:
EnvDTE.UIHierarchy solutionExplorer = dte.ToolWindows.SolutionExplorer;
object[] items = solutionExplorer.SelectedItems as object[];
EnvDTE.UIHierarchyItem item = items[0] as EnvDTE.UIHierarchyItem;
EnvDTE.ProjectItem projectItem = item.Object as EnvDTE.ProjectItem;
string path = projectItem.Properties.Item("FullPath").Value.ToString();